A French astronomer, Jean Dominique Cassini, saw a dark band running through the rings in 1675.  Later, it was found out that he had discovered a gap in Saturn’s rings.  The Cassini Division is made by two moons using their gravity to pull the rings toward each other.  Because the moons are on either side of the rings, it makes a gap.  The moons do not grab all the rings, so there are still some ringlets (tiny pieces of dust and other particles that make small rings) in the gap, escaping the pull of gravity.